Method of using a compensation mask to correct particle beam proximity-effect

ABSTRACT

Methods and apparatus for accurately performing proximity-effect compensation exposure are disclosed, even in cases where negative-type particle-beam-sensitive resist material is used. On a compensation mask, regions corresponding to regions on an underlying wafer that are to receive die patterns are subdivided into a multiple subfields having a pitch smaller than the spread width of particles back-scattered from the wafer 7. Certain of the subfields on the compensation mask define an aperture allowing passage therethrough of a particle beam. The aperture sizes are not uniform. Rather, each aperture is equal in area to an area of the nominally unexposed regions of the subfield less a prescribed constant area. The particle beam, after passing through a beam-shaping aperture, irradiates the compensation mask via an objective lens, and is scanned across the compensation mask by deflectors.

FIELD OF THE INVENTION

The invention pertains to particle-beam lithography methods andapparatus, especially such methods and apparatus employing electronbeams. More specifically, the invention pertains to methods andapparatus for reducing a so-called "proximity effect" arising from thescattering of particles from the beam in a particle-beam-sensitivesubstrate exposed to the charged particle beam.

BACKGROUND OF THE INVENTION

In particle-beam lithography apparatus and methods, a desired pattern iswritten or projection-exposed on a substrate such as a semiconductorwafer coated with a particle-beam-sensitive resist (hereinafter referredto simply as a "wafer"). Examples of particle beams include protons,ions, and electrons.

The most common particle beam used for such purposes is an electronbeam. The electron beam is made incident on the resist in a controlledway. Whenever electrons enter the resist, they lose energy andexperience trajectory changes via elastic and inelastic collisions knowncollectively as "electron scattering." Because scattered electrons canstill cause exposure-related changes in the resist, electron scatteringcan reduce the resolution of pattern linewidths or other desiredfeatures to be defined by the electron beam on the wafer. This effect isknown in the art as "proximity effect." The main type of scatteringresponsible for the proximity effect is back-scattering of electronsfrom the electron beam incident upon the electron-beam-sensitivesubstrate.

To compensate for the proximity effect, it is known in the art to makesubstantially equal, over the entire surface of theelectron-beam-sensitive substrate, the exposure of the substrate causedby these backscattered electrons.

To such end, it is known in the art to employ a "compensation mask"particularly with projection exposure of a particle-beam-sensitivesubstrate using a particle beam. In such a scheme, the pattern to betransferred (as defined on a mask or "reticle") to the substrate isdivided into many subfields. The compensation mask, which is separatefrom the reticle, is also divided into subfields (preferably in registerwith the subfields on the wafer). At each subfield on the compensationmask, an aperture through the thickness dimension of the mask is definedhaving an area that is equal to the area of the nominally unexposedregions of the corresponding subfield region on the substrate. With thismethod, by keeping the compensation mask and the substrate separated aprescribed distance from each other, uniform compensation exposure ofeach of the subfields is performed by blurring the energy beam(ultraviolet ray, electron beam, etc.) at the substrate by passage ofthe beam through the corresponding apertures in the compensation mask.

In the prior art described above, it is possible to favorably compensatefor proximity effect whenever a positive resist is used. However,whenever a negative resist is used, it is not possible to achieve highprecision of the pattern transferred to the wafer, despite attempts tocompensate for proximity effects. More specifically, with a positiveresist, if the compensating energy strikes a nominally unexposed region,there is only a small reduction in the resist-film thickness at thatregion, with no adverse effect on pattern precision. In contrast, with anegative resist, if the compensating energy strikes a nominallyunexposed region, some residual resist--a negative resist defect--canremain after developing the resist, making high-precision patternformation impossible.

Furthermore, with conventional proximity effect compensation methods,there is no particular mechanism for aligning the compensation mask andthe substrate. Such alignment has been performed, for example, with amicroscope or the like by viewing an open pattern at a prescribed areaon the mask and the corresponding pattern on the substrate. But, as thedegree of integration of transferred patterns has increased, it hasbecome desirable to increase the precision of alignment of thecompensation mask and the substrate, and perform compensation for theproximity effect in minute and close fashion in correspondence with thedegree of refinement in the pattern at the various areas on thesubstrate.

Compensation exposure of the entire surface of theparticle-beam-sensitive substrate has conventionally been conducted, forexample, by scanning an energy beam of prescribed width across thecompensation mask. Unfortunately, the precision by which such scans areplaced adjacent each other is typically poor, causing problematicunevenness in compensation exposure at boundary regions between scans.

SUMMARY OF THE INVENTION

In light of the foregoing, a key object of the invention is to provide aproximity-effect compensation method that will permit accuratecompensation for the proximity effect to be performed even when anegative-type particle-beam-sensitive material is used.

Another object of the invention is to provide a proximity-effectcompensation method that will permit compensation of the proximityeffect to be performed with accurate alignment between the compensationmask and the specimen (particle-beam-sensitive substrate) on whichcompensation is being performed.

Yet another object of the invention is to provide a proximity-effectcompensation method that will permit accurate compensation for theproximity effect to be achieved, even when compensation exposure isperformed by scanning with an energy beam and the precision of energybeam-seams at the boundary regions of scanned regions deteriorates.

According to a first aspect of the invention, an improvedproximity-effect compensation method is provided wherein an energy beam(e.g., electron beam, ion beam, or beam of ultraviolet light) isdirected toward a specimen (e.g., wafer) having aparticle-beam-sensitive resist coating. A compensation mask is placedbetween the specimen and the energy-beam source. The compensation maskcomprises multiple subfields each of which is smaller in width than thespread-width of energy backscatter in the specimen. At least some of thesubfields each define a respective aperture through which the energybeam must pass in order to reach the corresponding subfield on theparticle-beam-sensitive resist.

Preferably, each of the apertures has a transverse profile defining anarea corresponding, for the respective subfield on the resist, to anarea of nominally unexposed regions less a constant. The constant ispreferably equal to the area of nominally unexposed regions in asubfield on the resist having the most densely packed features. Suchregions tend to have the narrowest line widths (narrowest criticaldimensions). Thus, there is substantially no compensation exposure insubfields having the highest feature density. Consequently, even when anegative-type particle-beam-sensitive material is used, there ispractically no evidence of a so-called "scumming" phenomenon atnominally unexposed regions. This permits high-resolution formation ofhigh-density patterns. In other subfields having less feature density,compensation exposure occurs because the corresponding subfields on thecompensation mask are provided with apertures.

The energy beam irradiated on the specimen preferably exhibits ablurring that is no wider on the specimen than the width of thesubfields on the compensation mask. This facilitates a uniformcompensation exposure of the subfields on the specimen.

According to another aspect of the invention, the compensation maskpreferably comprises alignment marks that can be aligned withcorresponding alignment marks on the specimen so as to achieveregistration of the compensation mask with the specimen. Furtherpreferably, each set consisting of an alignment mark on the compensationmask and a corresponding alignment mark on the specimen is observedusing an optical lens operable to conjugate the compensation mask andthe specimen. The optical lenses are preferably mounted on a stageoperable to support the compensation mask and the specimen duringcompensation irradiation.

If, before initiating compensation irradiation, any misalignment isdetected between the alignment marks on the compensation mask and thespecimen, an appropriate movement of the compensation mask relative tothe stage (translational and/or rotational) is made to achieve alignmentand thus registration of the compensation mask and the specimen.

According to yet another aspect of the invention, the subfields on thecompensation mask are preferably smaller in width than the back-scatterspread width on the particle-beam-sensitive resist. The width of theenergy beam in a prescribed direction (e.g., the Y direction) incidenton the compensation mask is preferably an integral multiple of thewidth, in the same direction, of a die pattern on the specimen. Theenergy beam is preferably swept at a constant velocity in a directionorthogonal to the prescribed direction (e.g., swept in the X direction)on the compensation mask.

According to another aspect of the invention, an apparatus is providedfor performing backscatter-compensation exposure of a specimen. Theapparatus preferably comprises a source operable to produce an energybeam to which a particle-beam-sensitive resist, on the specimen, issensitive; a specimen stage, and a compensation mask. The specimen stageis operable to support the specimen during backscatter-compensationexposure. The compensation mask comprises multiple subfieldscorresponding to subfields on the specimen. At least some of thesubfields on the compensation mask each define a respective aperturesuch that the energy beam from the source must pass through an apertureto reach a corresponding subfield on the specimen. Each subfield on thecompensation mask is preferably smaller in width than the spread-widthof energy backscatter in the specimen. Finally, each of the apertures onthe compensation mask preferably defines an area corresponding, for therespective subfield on the specimen, to an area of nominally unexposedregions less a constant.

The foregoing and additional features and advantages of the presentinvention will be more readily apparent from the following detaileddescription, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic optical diagram showing relevant features of aproximity-effect compensation apparatus according to a preferred exampleembodiment of the invention.

FIG. 2 is a schematic plan view showing certain features of acompensation mask as used in the FIG. 1 embodiment.

FIG. 3 is an enlarged schematic plan view of a portion of thecompensation mask FIG. 2, showing subfields and apertures.

FIG. 4 is an enlarged schematic plan view showing an alignment mark onthe compensation mask in register with a corresponding alignment mark onthe wafer.

FIG. 5 is an enlarged schematic plan view showing exemplary patternfeatures transferred onto the wafer.

FIG. 6 is an enlarged schematic plan view showing a portion of acompensation mask, with subfields and apertures overlying the pattern ofFIG. 5.

FIG. 7 is an enlarged schematic plan view showing an aperture formed ata subfield on the periphery of the compensation mask.

DETAILED DESCRIPTION

FIG. 1 shows a preferred embodiment of a proximity-effect compensationapparatus, according to the invention, especially configured for usewith an electron beam. The FIG. 1 embodiment represents the current bestmode of an apparatus and method according to the invention.

In FIG. 1, during compensation exposure, a "cross-over" (i.e., image ofthe electron-beam source) 1 is formed by an electron beam emitted froman electron gun (not shown). The electron beam EB from the crossover 1passes through a beam-shaping aperture 2 that defines the transverseprofile of the electron beam. Below the beam-shaping aperture 2 thereare arranged in sequence an objective lens 3 comprising anelectromagnetic lens, a main deflector 4, a sub-deflector 5, and acompensation mask 6. A wafer 7, serving as the specimen upon whichproximity-effect compensation will be performed and that is coated withan electron-beam resist, is situated at a prescribed axial distance fromthe compensation mask 6. The electron-beam resist is normally a negativeresist.

In FIG. 1, for purposes of description, the Z axis is parallel to anoptical axis AX of the objective lens 3, the X axis is parallel to theplane of the page, and the Y axis perpendicular to the plane of thepage. Thus, the X and Y axes define a plane perpendicular to the Z axis.

During compensation exposure, the electron beam EB, having passedthrough the beam-shaping aperture 2, is incident on a radiation region15 of the compensation mask 6 via an objective lens 3. (The radiationregion 15 on the compensation mask 6 usually corresponds with arespective die (not shown) on the wafer 7.) The beam-shaping aperture 2and the compensation mask 6 are arranged in conjugate relationship withrespect to the objective lens 3. Not intending to be limiting, FIG. 2shows that the beam-shaping aperture 2 imparts a rectangular transverseprofile to the electron beam EB and thus to the radiation region 15.(The rectangular profile of the radiation region 15 has a width LX inthe X direction and length LY in the Y direction.) However, theradiation region 15 can have another profile, such as a square. Usually,the profile of the radiation region 15 is determined by the profile ofthe respective dies on the wafer.

The electron beam EB is deflected in a first direction from the opticalaxis AX by the main deflector 4, and in a second direction, oppositefrom the first direction, by the sub-deflector 5. As a result, theelectron beam EB is perpendicularly incident upon the surface of thewafer 7 during compensation exposure. The main deflector 4 and thesub-deflector 5 are operable to sweep the electron beam EB across thecompensation mask 6 in the X and Y dimensions. (In FIG. 1, the electronbeam EB is swept across the compensation mask 6 in the X dimensionradiation-region by radiation-region; after a row of radiation regionsin the X dimension are scanned, the electron beam is moved a distance LYin the Y dimension to scan a new row of radiation regions in the Xdimension.) As stated above, the electron beam EB is swept in the Xdimension preferably at a constant velocity.

The principal ray of the electron beam EB is deflected in the Xdirection along a trajectory 16 (represented by a dashed line in FIG.1). If the angle of the trajectory 16 relative to the optical axis AX ata time t is θ, then the current IA(t) at the main deflector 4 and thecurrent IB(t) at the sub-deflector 5 at time t are respectively definedin terms of a prescribed coefficient I_(o) as follows (where t is thetime required for the electron beam EB to be scanned from a location onthe optical axis AX to a location corresponding to the maximumdeflection angle):

    IA(t)=I.sub.0 (θ/tan θ)t                       (1)

    IB(t)=-I.sub.0 (θ/tan θ)t                      (2)

The wafer 7 is mounted on a movable wafer stage 8, which is supportedand moved by a stage drive 10. The compensation mask 6 is also mountedon the wafer stage 8 via a compensation-mask stage 9. Thecompensation-mask stage 9 is operable to allow positional adjustment ofthe compensation mask 6, relative to the wafer stage 8, in the X and Ydimensions as well as rotationally about the optical axis AX. The waferstage 8 is operable to move the wafer 7 independently of the objectivelens 3 and deflectors 4, 5.

As is known in the art of electron-beam projection lithography, areticle defines a die pattern to be transferred to the surface of thewafer 7. On the reticle, the pattern is divided into multiple subfieldsthat are individually projected via the electron beam onto the surfaceof the wafer in an ordered manner to form each corresponding die patternon the wafer surface. Each subfield on the reticle represents a portionof a pattern to be transferred from the reticle to the wafer 7, thepattern being defined by regions that block or scatter electrons in thebeam and other regions that pass the particles substantially withoutblocking or scattering them. Different regions of the pattern on thereticle need not have the same pitch.

The compensation mask 6 has multiple subfields each preferablycorresponding to a respective subfield on the reticle and a respectivesubfield on the wafer 7. Each of certain subfields (but not necessarilyall subfields) on the compensation mask 6 defines an aperturetherethrough. The compensation mask 6 is situated a sufficiently shortaxial distance from the wafer 7 such that the electron beam EB does notdiverge, after passing through an aperture on the compensation mask 6,more than the width of the corresponding subfield on the wafer.

The compensation mask 6 also comprises alignment marks (registrationmarks) 11A and 11B. Each alignment mark 11A, 11B is preferably across-shaped open pattern (FIG. 4) and is situated outside any of thesubfields on the compensation mask 6. The alignment marks 11A, 11B areoptically detectable. Corresponding cross-shaped alignment marks 12A and12B, respectively, are situated on the wafer 7. The alignment marks 12Aand 12B are sized so as to respectively fit within the alignment marks11A and 11B (FIG. 4).

Optical lenses 13A and 13B are provided for conjugation of the surfacesof the compensation mask 6 and the wafer 7. The lenses 13A, 13B arerespectively arranged between the alignment marks 11A, 11B on thecompensation mask 6 and the alignment marks 12A, 12B on the wafer 7. Thelenses 13A, 13B are respectively secured to the side walls of themovable stage 8.

As can be understood from the above, production of the desired patternon the wafer is normally performed by a "pattern-transfer" exposure thatis separate from the "compensating" exposure intended to compensate forproximity effects. The compensating exposure can be performed before orafter the pattern-transfer exposure. In any event, the pattern-transferexposure and the compensating exposure should not exceed a prescribedcumulative exposure level for the resist.

A representative compensation mask 6 is shown in FIG. 2, showing aregion 22A corresponding to a region on the wafer 7 that receives thepattern-transfer exposure (i.e., the region filled by multiple diesplaced side by side on the wafer). The normal extent of "back-scatter"on the wafer 7 is indicated by the circle 23 having a diameter φ, whichis used to define a backscatter border 22B of width W around thepattern-transfer exposure region 22A. The width W is not less than φ/2.For example, the width W may be about 40 μm. The region 24 on thecompensation mask 6 encompassed by the regions 22A, 22B is dividedvertically and horizontally into subfields 25₁, 25₂, 25₃, . . . , havinga pitch P. Each such subfield is smaller than φ/2. For example, when thediameter φ is approximately 40 μm, the pitch P may be set at 3 μm.

As shown in FIG. 3, certain of the subfields (e.g., 25₁, 25₂, 25₃) havecorresponding apertures 26₁, 26₂, 26₃, that allow passage therethroughof the particle beam. Thus, multiple apertures are provided within theregion 24 on the compensation mask 6 in the X dimension and in the Ydimension. It will be noted, however, that certain other subfields(e.g., subfield 25_(i) in FIG. 3) on the compensation mask 6 lackapertures.

The sizes of the apertures, and whether or not an aperture is present ina subfield, are determined as discussed below with references to FIGS.5-7. A portion of an exemplary pattern transferred to the surface of thewafer 7 is shown in FIG. 5. Features 27, 28, and 29, indicated byhatching, are defined on a reticle and transferred byprojection-exposure using a particle beam to the surface of the wafer,preferably without irradiating intervening regions between the features.However, due to back-scatter, regions outside (e.g., between) thefeatures 27, 28, 29 also receive radiation during the pattern-transferexposure. The magnitude of this "back-scatter" irradiation tends to berelatively large at the periphery of features that receive a largeamount of pattern-transfer irradiation; i.e., in regions of the patternwhere the feature density is high. For example, in FIG. 5, moreback-scatter will occur at the periphery of each of the more denselyarranged features 28 than at the periphery of each of the more dispersedfeatures 27. Similarly, more back-scatter will occur at the periphery ofeach of the features 29 than at the periphery of each of the features28. As can be seen, the more densely the features are situated relativeto each other, the narrower the linewidths between the features. Tocompensate for differing degrees of proximity effects arising with suchdiffering "packing densities" of features, the magnitude of thecompensation exposure is made relatively large in subfields comprisingthe features 27, and relatively small in subfields comprising thefeatures 29.

FIG. 6 shows an example of how each aperture (if present) in arespective subfield on the compensation mask 6 is sized relative to thepacking density of pattern features in each subfield. The features 27,28, 29 are as shown in FIG. 5, and the hatched area represents thecompensation mask 6 overlying and in register with the subfields. Thesubfields have a pitch P (e.g., 3 μm). As can be seen, certain subfieldshave a corresponding aperture (non-hatched; e.g., apertures 31, 32) andothers do not. Each of the apertures shown also preferably has a shapecorresponding to the shape of the corresponding subfield (e.g. square asshown). The aperture 31 is relatively large and is situated in asubfield on the compensation mask 6 corresponding to a region in whichthe feature density is relatively low (a subfield comprising thefeatures 27). The aperture 32, on the other hand, is relatively smalland is situated in a subfield corresponding to a region in which thefeature density is moderate (a subfield comprising mostly the features28). No aperture is situated in subfields corresponding to regions inwhich the feature density is high (subfields comprising the features29).

Further with respect to FIG. 6, a reference square 30 is situated in asubfield containing the most densely packed features 29 of the pattern.The area of the reference square 30 corresponds to the total area of thenominally unexposed regions in that subfield (i.e., the total area,within the subfield, not occupied by actual features). In determiningthe size of apertures (if any) in other subfields, the area of thereference square 30 is subtracted from the sum, for each subfield, ofthe areas of nominally unexposed regions. If the area of the referencesquare 30 is equal to the area of nominally unexposed regions in asubfield, then that subfield is not provided with a correspondingaperture on the compensation mask. If the area of the reference square30 is less than the area of nominally unexposed regions in a subfield,then that subfield is provided with an aperture having an area equal tothe difference in the areas.

Although, in FIG. 6, the apertures 31, 32 are shown in the center of thecorresponding subfields, each aperture is preferably centered at alocation within the corresponding subfield representing the center ofmass of the respective nominally unexposed regions in the subfield.

Turning again to FIG. 2, compensation exposure is great at subfieldsoverlying the back-scatter border 22B. In such subfields (e.g., subfield25_(j) in FIG. 7), if an aperture is indicated, the aperture 33 (FIG. 7)preferably has a rectangular shape to facilitate escape to the peripheryof heat generated by the particle-beam radiation. In the compensationmask 6, the alignment marks 11A, 11B are preferably situated within suchsubfields.

The apertures on the compensation mask 6 are preferably formed using anelectron-beam. With respect to an 8-inch wafer, the number of 3 μm×3 μmsubfields is determined to be approximately 36.3×10⁸, as follows:

    π· 102/(3×10.sup.-3)!.sup.2 =36.3×10.sup.8

Each aperture at each subfield on the compensation mask 6 receiving anaperture is formed by a single pulse of the electron beam.

Referring now to FIG. 1, with the compensation mask 6 mounted on thecompensation-mask stage 9, the wafer stage 8 is moved via the stagedrive 10. Alignment of the compensation mask 6 with the wafer 7 ispreferably performed using a registration-measurement device oranalogous device situated outside the housing for the electronic lensesand deflectors. Preferably, for example, alignment in the X and Ydimensions of the alignment mark 11A with the alignment mark 12A ismeasured using a dark-field-illumination optical microscope 14A. Theoptical lens 13A, attached to movable stage 8, is used to facilitatesuch measurement. Another dark-field-illumination optical microscope 14Bis provided to measure alignment of the marks 11B and 12B (asfacilitated by the optical lens 13B).

FIG. 4 shows a representative image, as obtained using adark-field-illumination optical microscope, of the alignment marks 11A,12A aligned with each other. The image 12AM of the alignment mark 12A,produced via the optical lens 13A, is situated within the alignment mark11A. Whenever both marks 11A, 11B are aligned with their correspondingmarks 12A, 12B, respectively, in the manner shown in FIG. 4, thecompensation mask 6 is properly aligned with the wafer. Any requiredadjustment of the compensation mask 6 to achieve such alignment is madeby corresponding motions of the compensation-mask stage 9 in the Xdimension, Y dimension, and rotationally about the axis AX. As a result,for example, the subfields 25₁, 25₂, 25₃, . . . , on the compensationmask 6 (FIG. 2) are respectively positioned over the correspondingsubfields on the wafer 7.

The shape of each irradiation region 15 is determined, at least in part,by the shape of each die on the wafer. In FIG. 2, the main exposureregion 22A represents a region on the wafer 7 in which multiple dies(each die containing an entire "chip" circuit pattern) will be exposedside by side. By way of example, suppose that the shape of each die is arectangle of 30 mm in the X dimension and 20 mm in the Y dimension. Eachradiation region 15 of the compensation mask 6 would have a dimension LXof 10 mm and a dimension LY of 20.5 mm. (The compensation mask isilluminated or irradiated by scanning or sweeping the beam along the Xdimension, and the cross-sectional area of the beam along the Xdimension is about 10 mm in the X dimension and about 20 mm in the Ydimension.) The dimension LY is set to be roughly equal to theY-dimension (the dimension in which the shorter of the rectangular sidesof the die extends) of each die on the wafer 7. During compensationexposure, while advancing the radiation region 15 at a pitch of 20 mm inthe Y dimension, the respective radiation regions 15 are swept at aconstant velocity in the X dimension on the compensation mask 6. As aresult, the entire surface of the compensation mask 6 is sequentiallyswept by the electron beam. The sweep velocity is set to achieve desiredcompensating exposure values. By following this procedure,back-scattering is made substantially uniform over the entire surface ofthe wafer 7, and favorable compensation of the proximity effect isachieved.

In the above example, compensation exposure is overlapped approximately0.25 mm in the Y dimension at the boundary regions between adjacent dieson the wafer 7. However, because there is no circuit pattern at theboundary regions between dies, this is not a problem. In any event, anyunevenness in compensation exposure is eliminated over the regions onthe wafer 7 actually occupied by dies. The LY dimension of the radiationregion 15 may also be an integral multiple (e.g., 2×) the Y dimension ofthe die pattern. This enables the boundary regions of the die to be madeto match boundary regions that would exist if compensation exposure wereperformed using a swept electron beam. Thus, when compensation exposureis performed using a swept electron beam, compensation of the proximityeffect can be advantageously and accurately performed at the interior ofeach of the die patterns, even if the areas swept by the electron-beamcannot be precisely aligned with each other. Further, because the sweepvelocity of the electron beam is preferably constant, there is nounevenness in compensation exposure in the sweep direction.

As described above with respect to the multiple subfields on thecompensation mask 6, any aperture that is formed is small to the extentthat the respective subfield corresponds to a subfield on the wafer 7having nominally unexposed regions that are small in area. There will beno apertures formed in certain subfields containing the most denselypacked features and/or narrowest linewidths. Because compensationexposure is not performed in such high-density subfields, even if anegative resist is used, linewidths of nominally unexposed regions donot narrow or exceed tolerance by the so-called scumming phenomenon.

Further with respect to FIG. 1, since alignment of the alignment marks11A, 11B on the compensation mask 6 with the marks 12A and 12B on thewafer 7 is performed using the optical lenses 13A, 13B, respectively,attached to the movable wafer stage 8, the apertures on the compensationmask 6 are accurately aligned with the corresponding subfields on thewafer 7. This permits accurate compensation exposure of the wafer 7.

A compensation-exposure apparatus having a simpler construction thaneither a projection-transfer apparatus or a scanning apparatus can beused to perform compensation exposure on the wafer 7. The preferredapparatus performs the compensating exposure of at least an entire dieper pulse of the electron beam. This provides a marked improvement inthroughput compared to apparatus that must iteratively transfer thecompensation pattern subfield by subfield.

Since practically no compensation exposure is performed at regions onthe wafer at which features are formed at high density, any furthernarrowing of narrow linewidths of nominally unexposed patterns caused bythe scumming phenomenon in cases where negative-typeparticle-beam-sensitive materials are used is eliminated. This allowsaccurate compensation of the proximity effect even when negative-typeparticle-beam-sensitive material is used.

While negative resist is discussed above in connection with thepreferred example embodiment, a positive resist can alternatively beused. Where a positive resist is used, compensation exposure accordingto the invention provides satisfactory reduction of proximity effects.Especially because a decrease in the scumming phenomenon at high-densityregions is possible according to the invention, circuit patterns can nowbe transferred with high resolution.

Although an electron beam was used to perform compensation exposure inthe preferred example embodiment, it will be understood that the generalprinciples disclosed above are also applicable to methods and apparatusemploying other energy beams, such as ion beams, ultraviolet rays, etc.

While the invention has been described in connection with exampleembodiments, it will be understood that the invention is not limited tothose embodiments. On the contrary, the invention is intended to coverall alternatives, modifications, and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. In a method for compensating a proximity effectarising in a particle-beam-sensitive resist, applied to a substrate,from energy backscatter in the substrate caused by an energy beamincident on the resist, an improvement, comprising:placing acompensation mask between the resist and a source of an energy beam towhich the resist is sensitive, the compensation mask comprising multiplesubfields at least some of which defining a respective aperture suchthat the energy beam must pass through an aperture to reach acorresponding subfield on the resist, each subfield on the compensationmask being smaller in width than the spread-width of energy backscatterin the substrate, each of the apertures having a transverse profiledefining an area for the respective subfield on the resist, wherein thearea defined by each of the apertures corresponds, for the respectivesubfield on the resist, to an area of nominally unexposed regions less aconstant.
 2. The method of claim 1, wherein the energy beam imparts ablurring on the resist that is no wider than the subfields on thecompensation mask.
 3. The method of claim 1, wherein the constant isequal to the area of nominally unexposed regions in a subfield on theresist having the most densely packed features.
 4. A method forcompensating a proximity effect arising in a particle-beam-sensitiveresist from energy backscatter in the resist caused by an energy beamincident on the resist, the method comprising the steps:(a) placing asubstrate, on which the resist is applied, relative to an energy-beamsource such that the energy beam, following a trajectory, can beincident on the resist; (b) aligning a compensation mask between theenergy-beam source and the substrate, the compensation mask comprisingmultiple subfields at least some of which defining a respective aperturesuch that the energy beam must pass through an aperture to reach acorresponding subfield on the resist, each subfield on the compensationmask being smaller in width than the spread-width of energy backscatterin the substrate, each of the apertures having a transverse profiledefining an area for the respective subfield on the resist, the areacorresponding to an area of nominally unexposed regions less a constant,the compensation mask being aligned with the substrate by aligningalignment marks on the compensation mask with corresponding alignmentmarks on the substrate; and (c) propagating an energy beam from theenergy-beam source toward the compensation mask so as to allow energyfrom the energy beam passing through the apertures to be incident oncorresponding subfields on the resist.
 5. The method of claim 4, whereinstep (b) is performed by viewing the alignment marks on the compensationmask and corresponding alignment marks on the substrate using an opticallens.
 6. The method of claim 4, wherein the constant is equal to thearea of nominally unexposed regions in a subfield on the resist havingthe most densely packed features.
 7. In a proximity effect compensationmethod in which a particle-beam-sensitive resist layer on a specimen isirradiated using an energy beam that must pass through a compensationmask before being incident on the specimen, an improvementcomprising:(a) providing the compensation mask with multiple subfieldscorresponding with subfields on the particle-beam-sensitive resistlayer, each subfield on the compensation mask being smaller in widththan the back-scattered electron spread width, at least some of thesubfields on the compensation mask each defining an aperture allowingpassage therethrough of an energy beam to which the resist is sensitive,each aperture having a size corresponding to an area, in the respectivesubfield on the resist layer, that is nominally unexposed to the energybeam; and (b) directing the energy beam at the compensation mask toexpose the subfields on the specimen, corresponding to aperturedsubfields on the compensation mask, to the energy beam passing throughthe respective apertures so as to provide a level of exposure of thespecimen at each respective subfield to the energy beam sufficient tocompensate for backscatter occurring during a projection-exposure of thespecimen; and (c) sweeping the energy beam at a constant velocity in adirection orthogonal to a width of the energy beam directed at thecompensation mask.
 8. The method of claim 7, wherein, in step (b), theenergy beam directed at the compensation mask has a width, in aprescribed direction, on the compensation mask that is an integralmultiple of the width, in the prescribed direction, of a die patternformed on the specimen.